Moisture and

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Chapter 4 Moisture and Atmospheric Stability
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hydrologic cycle

• hydrologic cycle (or water cycle). The cycle
  illustrates the continuous movement of water.

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Condensation

• Condensation occurs when air cannot hold any more
  water vapor.

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Evaporation

• is the vaporization of a liquid and the reverse
  of condensation.

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Transpiration

• is the evaporation of water from the aerial parts
  of plants, especially leaves but also stems,
  flowers and roots.

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Sublimation

• is a transition from the solid to gas phase with
  no intermediate liquid stage.

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Deposition

• is a process in which gas transforms into solid
  (also known as desublimation)

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Latent Heat

• the amount of energy released or absorbed by a
  chemical substance during a change of state that
  occurs without changing its temperature, meaning
  a phase transition such as the melting of ice or
  the boiling of water.

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Humidity

• Humidity is a measure of moisture in the air,
  specifically that of water vapor, which is water
  in its gaseous state. Find out how to measure
  absolute humidity and how to determine the ratio
  of dry air to water vapor.

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Vapor Pressure and Saturation

• is the pressure of a vapor in equilibrium with
  its non-vapor phases. All liquids and solids have
  a tendency to evaporate to a gaseous form, and
  all gases have a tendency to condense back into
  their original form

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Relative Humidity

• is a term used to describe the amount of water
  vapor that exists in a gaseous mixture of air and
  water.

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Water Vapor -- review

• Water vapor, an odorless, colorless gas, can
  change from one state of matter (solid, liquid or
  gas) to another at the temperatures and pressures
  experienced on Earth. The heat energy involved in
  the change of state of water is often measured in
  calories. The processes involved in changes of
  state include evaporation (liquid to gas),
  condensation (gas to liquid), melting (solid to
  liquid), freezing (liquid to solid), sublimation
  (solid to gas), and deposition (gas to solid).
  During each change, latent (hidden, or stored)
  heat energy is either absorbed or released.

Video clip
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Humidity -- review

• term used to describe the amount of water vapor
  in the air. The methods used to express humidity
  quantitatively include (1) absolute humidity, the
  mass of water vapor in a given volume of air, (2)
  mixing ratio, the mass of water vapor in a unit
  of air compared to the remaining mass of dry air,
  (3) vapor pressure, that part of the total
  atmospheric pressure attributable to its
  water-vapor content, (4) relative humidity, the
  ratio of the airs actual water-vapor content
  compared with the amount of water vapor required
  for saturation at that temperature, and (5) dew
  point, the temperature to which a parcel of air
  would need to be cooled to reach saturation.

Video clip
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Saturated Air

• When air is saturated, the pressure exerted by
  the water vapor, called the saturation vapor
  pressure, produces a balance between the number
  of water molecules leaving the surface of the
  water and the number returning. Because the
  saturation vapor pressure is temperature-dependent
  , at higher temperatures more water vapor is
  required for saturation to occur.

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Hot desert air with low relative humidity will
actually have a higher water vapor-content than
frigid air with high relative humidity
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Relative Humidity -- review

• Relative humidity can be changed in two ways (1)
  by changing the amount of moisture in the air or
  (2) by changing the airs temperature. Adding
  moisture to the air while keeping the temperature
  constant increases the relative humidity.
  Removing moisture lowers the relative humidity.
  When the water vapor content of air remains at a
  constant level, a decrease in air temperature
  results in an increase in relative humidity, and
  an increase in temperature causes a decrease in
  relative humidity.

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Changing RH

• In nature there are three major ways that air
  temperatures change to cause corresponding
  changes in relative humidity (1) daily (daylight
  versus nighttime) changes in temperature, (2)
  temperature changes that result as air moves
  horizontally from one location to another, and
  (3) changes caused as air moves vertically in the
  atmosphere.

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Dew Point -- review

• An important concept related to relative humidity
  is the dew-point temperature (or simply dew
  point), which is the temperature to which a
  parcel of air would need to be cooled to reach
  saturation. Unlike relative humidity, which is a
  measure of how near the air is to being
  saturated, dewpoint temperature is a measure of
  the airs actual moisture content. High dew-point
  temperatures equate to moist air, and low
  dew-point temperatures indicate dry air. Because
  the dew-point temperature is a good measure of
  the amount of water vapor in the air, it is the
  measure of atmospheric moisture that appears on
  daily weather maps.

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Instruments

• A variety of instruments, called hygrometers, can
  be used to measure relative humidity. One of the
  simplest hygrometers, a psychrometer, consists of
  two identical thermometers mounted side by side.
  One thermometer, called the wet-bulb thermometer,
  has a thin muslin wick tied around the bulb.

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Temperature Changes

• When air is allowed to expand, it cools. When air
  is compressed, it warms. Temperature changes
  produced in this manner, in which heat is neither
  added nor subtracted, are called adiabatic
  temperature changes.

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Rising air

• When air rises, it expands and cools
  adiabatically. If air is lifted sufficiently
  high, it will eventually cool to its dewpoint
  temperature, and clouds will develop. Four
  mechanisms that cause air to rise are (1)
  orographic lifting, where air is forced to rise
  over a mountainous barrier, (2) Frontal wedging,
  where warmer, less dense air is forced over
  cooler, denser air along a front, (3)
  convergence, a pile-up of horizontal airflow
  resulting in an upward flow, and (4) localized
  convective lifting, where unequal surface heating
  causes localized pockets of air to rise because
  of their buoyancy.

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Rising Air

• When air rises, it cools and can eventually
  produce clouds. Stable air resists vertical
  movement, whereas unstable air rises because of
  its buoyancy. The stability of air is determined
  by knowing the environmental lapse rate, the
  temperature of the atmosphere at various heights.
  The three fundamental conditions of the
  atmosphere are (1) absolute stability, when the
  environmental lapse rate is less than the wet
  adiabatic rate, (2) absolute instability, when
  the environmental lapse rate is greater than the
  dry adiabatic rate, and (3) conditional
  instability, when moist air has an environmental
  lapse rate between the dry and wet adiabatic
  rates. In general, when stable air is forced
  aloft, the associated clouds have little vertical
  thickness, and precipitation, if any, is light.
  In contrast, clouds associated with unstable air
  are towering and frequently accompanied by heavy
  rain.

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As long as air in a balloon is hotter than the
surrounding air, it will rise
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Instability of Air

• Any factor that causes air near the surface to
  become warmed in relation to the air aloft
  increases the airs instability. The opposite is
  also true any factor that causes the surface air
  to be chilled results in the air becoming more
  stable. Most processes that alter atmospheric
  stability result from temperature changes caused
  by horizontal or vertical air movements, although
  daily temperature changes are important too.
  Changes in stability occur as air moves
  horizontally over a surface having a markedly
  different temperature than the air. Furthermore,
  subsidence (a general downward airflow) generally
  stabilizes the air, while upward air movement
  enhances instability.

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Chapter 4

• END